Shock sensors employing reed switches are used in motor vehicles to detect vehicle collisions. When such a collision occurs, the shock sensor triggers an electronic circuit for the actuation of one or more safety devices. Devices which may be actuated include air bags, safety belt tensioners, fuel system shut offs, and radio signals. Shock sensors typically employ a reed switch and an acceleration sensing magnet. The magnet is typically biased by a spring against an abutment spaced from the reed switch such that the unaccelerated reed switch is not activated. When the vehicle and the reed switch which is rigidly attached to the vehicle are subject to a crash-induced acceleration, the magnet, acting as an acceleration-sensing mass, moves relative to the reed switch to a position which activates the reed switch.
The duration with which the reed switch in a shock sensor remains activated is important. The complexity of the circuitry required to detect activation and the reliability of the differentiation between switch activation and spurious noise is dependent on the time the switch remains closed --known as the switch dwell time. Longer dwell time permits simpler circuitry and greater detection reliability.
Known shock sensors employ mechanical delay mechanisms which retard return of the activation magnet to the unactivated position and thereby increase the duration of activation. Though quite effective, shock sensors employing mechanical delays result in shock sensors of somewhat greater mechanical complexity and size.
Other known shock sensors increase the time of switch activation by increasing the sensitivity of the contacts of the reed switch, or increasing the length of the activation magnet, to effect an increase in the size of the switch activating regions. However, increasing the length of the magnet and the distance which the magnet may travel while activating the reed switch increases the package size of the shock sensor and, further, does not address the problem of minimum dwell associated with a minimum crash situation.
Because placement of the shock sensor within the automobile may be critical to its reliable and effective operation, packaging size of the overall shock sensor is important, because a smaller sensor may be more readily mounted in a suitable location.
Switches having an extended minimum dwell time are advantageous in many situations where it is important to avoid ambiguity caused by extremely short minimum dwell or closure times.
What is needed is a shock sensor with extended dwell and extended minimum dwell, which is available in a physical package of small dimensions.